Don't send a resize event if the window size didn't change
auto merge of #551 : sfowler/servo/run-layout-once, r=metajack

Performance will be better and benchmarking easier if we don't run layout an extra unnecessary time when loading a page from the commandline. This is happening because we always send a resize event to the script task from the compositor whenever the platform tells us one happened, even if the window size didn't change. This PR fixes this by checking that the window size is actually different before sending the event.